We typically only go to one cg that is electric only, Knoebels Amusement Park where we fill at home. Rest of time 1/3 as others have stated. Fill using my hose and filter as I know where it's been.
As to why not fully fill for 2 reasons, 1 our tank is forward and it adds tongue wt and 2 we use so little water I'd rather not have 30 gallons setting around stagenet in the humid heat of summer here in the North East. YMMV